Normal spring biased bridging tap changer reciprocating contact assembly with longitudinally displaceable actuator means

ABSTRACT

A tap changer is housed within a vertically arranged rectangular frame structure and contains a plurality of spaced taps. The taps can be selectably engaged by a contact element through the medium of a handle rod which is movably displaceable within the frame structure along a rectilinear path. In the top portion of the frame structure a number of receiving slots, corresponding in number and spacing to the taps in the tap changer, are arranged perpendicularly to the rectilinear path of the handle rod so that the handle rod can be inserted selectably into the slots in effecting contact between the contact elements and a selected one of the taps. The handle rod is connected to the contact elements through a link arrangement positioned within the frame structure and the link arrangement includes springs for biasing the contact elements into contact when a circuit is closed and for withdrawing the contact element when a circuit is opened. Due to the link arrangement, as the contact element is moved into a closed position a wiping action is provided with the taps. Movement of the handle rod between the receiving slots can only be effected when the contact element is in an open-circuit state.

[451 Mar. 12, 1974 United States Patent [191 Shirae NORMAL SPRING BIASED BRIDGING TAP Primary Examiner-James R. Scott CHANGER RECIPROCATING CONTACT ASSEMBLY WITH LONGITUDINALLY DISPLACEABLE ACTUATOR MEANS Attorney, Agent, or FirmToren, McGeady and Stanger [57] ABSTRACT A tap changer is housed within a verticall rectangular frame structure and contains a [76] Inventor: Shinichi Shirae, 4-3, Chuo 6-ch0me,

Tokyo, Japan y arranged plurality of [22] Filed: Apr. 18, 1973 App]. No.: 352,335

spaced taps. The taps can be selectably engaged by a contact element through the medium of a handle rod June 7, Ser. No.

Related U.S. Application Data [63] Continuation-impart of Ser. No 260,701

1972, abandoned, which is a continuation of 134,716, April 16, 1971, abandoned.

ear path of the handle rod so that the handle rod can be inserted selectably into the slots in effecting contact between the contact elements and a selected one of the taps. The handle rod is connected to the contact elements through a link arran gement positioned within the frame structure and the link arrangement includes springs for biasing the contact elements into contact when a circuit is closed and for withdrawing the contact element when a circuit is o O G 3 36 5 5 mmR Am 0 A 13 C8 ma /3 1 D lR m W ,wO 2 I m..E l m m .1 0 n u m M ."h6 nuc r/ mmwm WM Z .L h C smfim UIF ll] 2 8 555 [56] References Cited pened' as the contact element 15 UNITED STATES PATENTS moved into a closed position a wipin vided with the taps. Movement of th to the link arrangement,

g action is proe handle rod between the receiving slots can only be effected when 200/16 E the contact element is in an open-circuit state. 200/18 X CX 6E 1 l8 0 OH 20 .0 2

1124,586 1/1959 Davis 2,896,707 7/1959 Jennings....... 3,187,120 6/1965 3,467,922 9/1969 Greenwall, Jr. et a].

10 Claims, 5 Drawing Figures NORMAL SPRING BIASED BRIDGING TAP CHANGER RECIPROCATING CONTACT ASSEMBLY WITH LONGITUDINALLY DISPLACEABLE ACTUATOR MEANS This application is a continuation-in-part of application Ser. No. 260,701, filed June 7, 1972, now abandoned, which is a continuation of application Ser. No. 134,716, filed Apr. 16, 1971, now abandoned.

SUMMARY OF THE INVENTION This invention relates to a tap changed having a fixed collector contact, a plurality of fixed tap contacts and a movable contact which is adapted to selectively electrically connect a desired one of the tap contacts to the collector contact.

The conventional tap changed has been, in many cases, constructed with a rotating axis and plural taps arranged radially around the rotating axis. In order to increase the number of taps on such tap changer, it is necessary to increase the diameter of the circle of the taps. In addition, the conventional tap changer is accompanied with much unavailable space. Because ofits cylindrical configuration, four useless corner spaces have been left in the installation of the device.

Furthermore, the conventional tap changer of the above type has an electric passage for a mechanically rotating portion or flexible wire and it is well known that it has low persistence and is dangerous for handling, because of mechanical abrasion or exhaustion.

Moreover, the conventional tap changer is not capable of a direct contacting operation of the contact element with an aimed tap without any contact with other interposed taps at the tap selecting operation. Accordingly the operation of the conventional tap changer requires considerable labour and time and involves the rapid wearing of the contact element.

It is, therefore, a primary object of the present invention to provide an improved tap changer in which all the disadvantage of the conventional tap changer as above mentioned are eliminated.

It is another object of the present invention to provide such a thin shaped tap changer that it can be suitably arranged even in a narrow space. The addition of the number of taps can be achieved only by the modification of the length. Change in the width and the depth of a space is not necessary.

Further object of this invention is a tap changer in which the bridge contact and the fixed contacts are wiped by each other when the tap changer is closed so that the contacting surface may be maintained clean free from dust or dirt.

These and other objects of this invention can be accomplished by the tap changer comprising a fixed collector contact, a plurality of fixed tap contacts and a movable contact which is adapted for selectively electrically connecting a desired one of the tap contacts to the collector contact, characterized in that the tap contacts are mounted in a linear row in spaced relationship on an electrically insulating base of a housing. The collector contact is composed of an electrically conductive elongated strip mounted on the insulator base in opposition to and flush with the row of the tap contacts electrically isolated from the collector contact. The movable contact is supported over the collector contact and the tap contacts by means of a framework which, in turn, is supported movably toward and away from the base by a supporting member. The

supporting member is movable over and along the row of tap contacts or over the length of the elongated collector contact when the tap changer is in the open position. Toggle means is provided on the supporting member to move the framework and thereby cause the movable contact to contact simultaneously the collector contact and the selected one of the tap contacts in a bridge like manner.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated and described preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWING Now the invention will be described in detail with reference to a preferable embodiment shown in the drawings, in which FIG. 1 is a perspective view of a tap changer according to the present invention;

FIG. 2 is a rear view of the same;

FIG. 3 is a sectional view taken along the line III-lll in FIG. 2 showing the tap changer in an open position;

FIG. 4 is a view similar to FIG. 3 but showing the tap changer in the closed position; and

FIG. 5 is a sectional view taken along the line VV in FIG. 2 to show the essential parts of the tap changer in the closed position.

DETAILED DESCRIPTION OF THE INVENTION In FIG. 1, a tap changer embodying the present invention and containing a number of tap positions, includes a housing 1 which comprises a horizontally arranged bottom plate 2 formed of an electrically insulating material having a thickness suitable to ensure the rigidity of the structure. Fixedly secured to the opposite ends of the bottom plate 2 are vertical walls 3 and 4. The upper ends of the walls 3, 4 are fixedly interconnected by a top cover plate 5 provided with a plurality of equispaced receiving slots 6 along an outer edge thereof at the positions corresponding to the tap positions. Additionally, the side walls 3 and 4 are connected to each other below the top cover plate 5 by a transversely extending vertical plate 7 of a substantially rectangular form, the upper and lower edges of the plate are bent inwardly to form guide rails 8 and 9 disposed in opposition and in parallel to each other as illustrated in FIGS. 1 and 3 to 5. Supported laterally slidably between the guide rails 8 and 9 is a plate 10 which is arranged to support a tap changer mechanism, as hereinafter described. The supporting plate 10 has side walls 11 integrally formed at and extending along both of its vertical sides. Each of the side walls 11 is provided with oppositely disposed guide notches 12 and 12 at the upper and lower ends, respectively. As displayed in FIGS. 3 and 4, the lower notch 12' has a greater width than the upper notch 12. The guide rails 8 and 9 engage slidably with these notches 12 and 12' so that the supporting plate 10 may be moved laterally over the tap positions.

Secured to the plate 10 by the way of an intermediate spacer plate 13, note FIGS. 3 and 4, is a fixed arm member 14 which supports a horizontally extending shaft which in turn pivotably or swingably supports a bifurcated link lever 16. The link lever 16 consists of a pair of laterally spaced legs 16a and 16b which are integrally connected to each other at their upper ends by a web 16c. A handle rod 17 is fixed to and extends upwardly from the web 160 and has a manually operable knob 18 fixed to its upper end. It will be understood that the width of the slots 6 formed in the cover plate 15 are dimensioned so that they can receive the handle rod 17.

The arm member 14 has a downwardly extending portion 19 of a substantially rectangular configuration, note FIG. 1. This extension portion 19 serves as a guide for a vertically movable framework 20 which carries a movable bridge contact assembly in a suspending manner as will be hereinafter described. The lower end of the extension portion 19 is bent rearwardly toward the supporting plate 10 and a space 24 is formed between the rear surface of the extension portion 19 and the front surface of the supporting plate 10 (FIG. 4-). The framework 20 has a chairlike structure and consists of a pair of upright side plates 21 and 22 of substantially the same configuration and an integrally formed connecting web 23, see FIG. 3, which is freely disposed within the space 24 between the extension portion 19 and the supporting plate 10. In this manner, the framework generally indicated by reference numeral 20 can be vertically moved up and down guided by the extending portion 19. 4

A second shaft 27 extends horizontally through both side plates 21 and 22 and a pair of levers 28, only one of which can be seen in FIG. 1, are pivotably mounted on the shaft 27. The other ends of the levers 28 from the shaft 27 are linked to the legs 16a and 16b, respectively, of the bifurcated lever 16 by means of a shaft 29 located above shaft 27. The shafts 27 and 15 are connected to each other by a pair of tension springs 30 usually exerting a biasing force for pulling upwardly on the shaft 27 and hence on the framework 20.

Referring again to FIG. 1, a collector contact 31, which is in a form of an Lshaped metal plate and adapted to be connected to a bus bar, not shown, is secured to the insulated bottom 2 along a longitudinal side thereof. Located on the opposite longitudinal side of the bottom is a plurality of fixed tap contacts 32 electrically and geometrically isolated from one another. These tap contacts 32 are vertically aligned with the slots 6 formed in the top cover 5 and are adapted to be connected to different potential sources such as taps of a secondary winding of a voltage regulator transformer, not shown. It should be noted that the individual tap contacts 32 are formed of L-shaped barlike metal plates. The selective electrical connection between the fixed collector contact 31 and one of the tap contacts 32 is effected by a movable bridge contact 33, as shown in FIGS. 3 and 4. The movable bridge contact 33 is fixed to an insulator block 341 of an inverted T-shaped cross section at the lower surface thereof by means of a screw 35. A longitudinal groove 36 formed in the insulator bottom plate 2 which geometrically separates the contacts 31 and 32 from each other also serves to accomodate the screw when the bridge contact 33 is in the closed position, note FIG. 4. In this connection, it will be seen in FIG. 4 that the L- shaped collector contacts 31 and tap contact 32 form in a sense a loop-like current path in co-operation with the movable bridge contact 33. It has been found that such an arrangement as well as the configuration of the contacts 31, 32 and 33 are advantageous in that an are or spark, which is likely to be produced in opening the electrical connection, can easily be blown out by a magnetic field which is built around the contacts and tends to expel the arc outwardly as viewed in FIG. 4.

Fixed on the insulator block 34 at the top surface thereof is a metal plate 37 of a rectangular shape having a greater width than that of the protrusion or upwardly extending leg of the insulator block 34 so that the movable contact assembly consisting of the bridge contact 33, insulator block 34 and the top metal plate 37 may be carried or suspended from the framework 20. To this end, the side plates 21 and 22 of the framework are bent at the lower ends thereof inwardly at substantially right angle so that the bent ends 21 and 22 lie below the projecting side edges of the plate 37, as shown in FIG. 2.

Referring to FIGS. 1 and 5, a spring seat plate 25 is pivotably supported on the shaft 27 between the side plates 21 and 22 of the framework 20. A compression spring 26 is disposed between the underside of the spring seat plate 25 and the upper surface of the metal plate 37. I

It is important to note that the vertical length or height of the side plates 21 and 22, Le, the distance between the shaft 27 and the inwardly bent lower end of the side plates 21 and 22 of the framework 20, is dimensioned so that, in the open position of the tap changer, the inwardly bent lower ends of the side plates 21 and 22 will physically contact the lower surface of the plate 37 ,under the force of the compression spring 26, and thereby suspend the movable contact assembly away from the fixed contacts 31 and 32, while in the closed position of the tap changer, the inwardly bent lower ends of the frame plates 21 and 22 make no physical contact with the lower surface of the plate 37 whereby the movable bridge contact assembly (33, 34, 37) may freely be brought to the bridging contact position on the fixed contacts 31 and 32 under the influence of the spring 26.

According to another feature of this invention, a contact pressure balancing means is provided which includes a plate 38 having depending side walls for partially covering the upper portion of the bridge contact assembly. This plate 38 is connected at the rear end, as viewed in FIG. 3, to the bridge contact assembly by means of a bolt and nut 39 and has a rectangular opening 40 through which the compression spring 26 as well as the side plates 21 and 22 of the framework 20 extend, as can best be seen in FIG. 1. Each of the side plates 21 and 22 has a notch 41 or 42,-respectively formed at the front edge adjacent to and spaced above the plate 37. The front end portion of the plate member 38 is held by these notches 41 and 42 so that it inclines upwardly to the left as viewed in FIGS. 3 and 4. The cover plate 38 serves to retain the side plates 21 and 22 of the framework 20 in place and additionally to equalize the contact pressure of the bridge contact 33 onto the bar contact 31 and the tap contacts 32 as will be hereinafter described.

Referring to FIG. 1, a series of holes 43 are formed in the guide plate 7 along and adjacent to its longitudinal lower end. The holes 43 are in the vertical alignment with the tap contacts 32 and the slots 6 formed in the upper cover 5. These holes are arranged to receive a position indexing semispherical projection 44 mounted on the free end ofa leaf spring 45 which is se' cured at its other end to the rear surface of the slidable supporting plate 10.

Now the operation of the inventive tap changer will be described. It shouldbe understood that the bifurcated lever 16 and the link levers 28 articulated to it constitute a so-called toggle mechanism together with the tension springs 30 stretched between the pivotal shafts 15 and 27. The dead point of this toggle mechanism lies on the line or dead line passing through the shafts 15 and 27. The vertically movable framework 20, guided by the fixed guiding extension portion 19 and suspending or carrying the movable bridge contact 33, is connected to this toggle mechanism by means of the shaft 27. The tap changer is in the open position in FIG. 3 with the knob 18 as well as the actuator rod 17 tilted to the right out of the corresponding receiving or indexing slot 6. The articulation of the link lever 16 and levers 28 provided by the pivotal shaft 29 lies at the left side of the dead line of the toggle mechanism with the framework 20, as well as the bridge contact 33 suspended from it, displaced upwardly to the upper limit defined by the lower edge of the spacer plate 13 under the influence of the tension springs 30. Thus, the entire switching unit including the toggle mechanism, the vertical movable framework and the bridge contact assem bly can be moved laterally by means of the manipulating knob 18, wherein the supporting plate is guided by the guide plate 7 with the opposing guide rails 8 and 9 engaging the notches 12. When the switching unit reaches the position at which a desired tap contact is to be connected to the bus bar and which can be indexed by the co-operation of the hole 43 and the semispherical projection 44, the knob 18 and the actuator rod 17 are forcibly tilted into the corresponding receiv' ing slot 6 around the fixed shaft against the force of the tension springs 30 so that the framework 20 suspending the bridge contact assembly is lowered to the collector contact 31 and the selected tap contact 32. If the articulating point of the levers 16 and 28 which corresponds to the shaft 29 crosses the dead line or the longitudinal axis of the tension spring 30, the toggle mechanism automatically attains the position shown in FIG. 4 under the force of the tension springs 30, wherein the framework 20 is moved to the lowermost position and the bridge contact 33 is pressed into engagement with the fixed contacts 31 and 32 in a bridgelike manner under the influence of the powerful compression spring 26. It is to be noted that, during this closing phase, a very advantageous effect or action is brought about. In more detail, it should be recalled that the lower notch 12 is formed wider than the upper notch 12 and its width is greater than the thickness of the guide rail 9. Accordingly, when the actuator rod 17 is rotated in the counterclockwise direction starting from the open position shown in FIG. 3 to initiate the closing operation, the supporting plate 10 is at first swung in the counterclockwise direction around the upper guide rail 8 as far as the width of the lower notch 12 permits. By further rotating the rod 17 into the slot 6, the movable bridge contact 33 then comes into contact with the bus and tap contacts 31 and 32. It should be noted that the dimension of the levers 16 and 28 are so selected that such contact will take place before the articulated point or pivotal shaft 29 passes across the dead line. The continuation of the rotation of the actuator rod 17 thereafter will lower the framework 20 and cause the spring 26 to be compressed to increase the contacting pressure between the contacts 31, 32 and 33. When the pivotal shaft 29 has passed the dead line to the right as viewed in FIG. 3, the contacting pressure exerted by the compression spring 26 produces a horizontal component of force in the bridge contact assembly in the direction indicated by an arrow 50 in FIG. 4 due to the fact that the supporting plate 10 has been swung in the counterclockwise direction as explained above. This horizontal component of force of the compression spring causes the bridge contact 33 to move slidably or to be displaced in the left direction as viewed in FIGS. 3 and 4, to the balanced position at which only the vertical pressing force is exerted onto the bridge contact 33. As a result of this wiping action, the contacting surface of the various contacts are maintained clean, free of dust, dirt and the like which are likely to produce undesireable arcing or sparking and increases the contact resistance. The leaf spring 45 will contribute to the enhancement of this wiping action to some degree. In order to produce the smooth wiping action, the magnitude of the horizontal component as well as the vertical component of the force exerted onto the bridging contact 33 are to be the same at the collector and the tap contacts. This is assured by the cover member 38, without which it has been found that the pressing force of the bridging contact 33 onto the collector contact 31 tends to become greater than that on the tap contact 32 which in turn results in the wiping action or horizontal sliding movement of the bridge.

contact 33 being of no smoothness. The counterclockwise rotation of the articulated point 29 is stopped by the guiding edges 21a and 22a of the framework 20.

In the foregoing a preferable embodiment of the invention has been shown and described. However, it should be appreciated that this is only an example and many modifications and changes in detail will be easily conceivable for those skilled in the art without departing from the scope and spirit of this invention. For example, rolls or capstans may be used in place of the notches 12. Furthermore, as a guiding means for the vertical movement of the framework 20, a groove formed in the supporting plate 10 may be employed.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

What is claimed is:

l. A tap changer movable between an open position and a closed position comprising a fixed collector contact, a plurality of fixed tap contacts, and a movable contact arranged for selectively electrically connecting a desired one of said tap contacts to said collector contact, wherein the improvement comprises a housing, said housing including an electrically insulating base, said tap contacts mounted in a linear row in spaced relation on said electrically insulating base, said collector contact composed of an electrically conductive elongated strip mounted on said insulator base in spaced opposition to and flush with the row of said tap contacts and electrically isolated from said tap contacts, a framework for supporting said movable contact over said collector contact and said tap contacts, a supporting member movably supporting said framework so that it can be moved toward and away from said base, means supporting said supporting member so that it is movable over and along the row of said tap contacts for the length of said elongated collector contact when said tap changer is in the open posi tion, and toggle means mounted on said supporting member for biasing said framework to hold said movable contact in simultaneous contact, said collector contact and the selected one of said tap contacts in a bridge like manner.

2. Tap changer, as set forth in claim 1, wherein said housing supporting said elongated insulator base is in the form of a rectangular box and includes upstanding side walls fixed at their lower ends to said base and a top cover plate connected to the upper ends of said side walls, an upwardly extending transverse plate extending between and connecting said upstanding walls to each other, said transverse plate located between said base and said top cover plate and forming guides along its upper and lower edges, so that siad supporting plate can be moved and guided by said guides and at the same time is supported so that said supporting plate is swingable about the upper guide to some degree.

3. Tap changer, as set forth in claim 2, wherein said top plate is formed with a plurality of slots along a longitudinal side thereof with each said slot in vertical alignment with one of said tap contacts.

4. Tap changer as set forth in claim 2, wherein an indexing means is associated with said supporting plate and seats into a hole in said transverse plate and is provided to facilitate the positioning of said movable contact at the one of said tap contacts to be closed.

5. Tap changer, as set forth in claim 1, wherein an arm member secured to said supporting plate, a first shaft supported horizontally on said arm member, said toggle means comprising a bifurcated lever swingably mounted on said first shaft and a pair of link levers connected pivotably to the free end of said bifurcated levers, a second shaft supported horizontally on said framework, the opposite ends of said link levers from said bifurcated levers pivotably connected to said second shaft tension springs connecting said first and said second shafts together, and the axis of said tension springs forming a line on which the dead point of said toggle mechanism lies.

6. Tap changer, as set forth in claim 4 wherein a toggle mechanism actuating rod is connected to said bifurcating lever and projects upwardly above said top cover plate, said actuating rod being arranged to be received in said receiving slot in the closed position of the tap changer.

7. Tap changer as set forth in claim ll, wherein said framework comprises a pair of side plates disposed in spaced parallel relation, a web connected to said side plates and serving to guide the vertical movement of said framework along the surface of said supporting plate, means attached to said side plates and arranged to releasably suspend said bridge-like movable contact, a spring seat plate rotatably mounted on said second shaft between said side plates, and a compression spring vertically disposed between said seat plate and the upper surface of said-means suspending said bridgelike movable contact bridge contact assembly.

8. Tap changer, as set forth in claim 7, where the lower ends of said side plates are bent inwardly and the distance between said second shaft and said inwardly bent lower ends of said side plates is dimensioned so that, in the closed position of the tap changer, the suspension of said movable contact is released, whereby the said movable contact and its said suspension means assembly can be brought freely to the bridging contact position on said fixed contacts under the influence of said compression spring.

9. Tap changer as set forth in claim 7 characterized in that the longitudinal axis of said compression spring and the vertical plane of said supporting plate are swingable to some degree around the upper edge of said supporting plate and are positioned relative to each other so that a component of force is produced by said compression spring in addition to the force to press the bridge contact assembly onto said collector and tap contacts thereby to cause said movable contact to move over the collector and tap contacts in a wiping manner when the tap changer is closed.

10. Tap changer as set forth in claim 9, wherein a force balancing member is associated with said movable contact to equalize the pressure of the spring compression at said collector and the tap contacts. 

1. A tap changer movable between an open position and a closed position comprising a fixed collector contact, a plurality of fixed tap contacts, and a movable contact arranged for selectively electrically connecting a desired one of said tap contacts to said collector contact, wherein the improvement comprises a housing, said housing including an electrically insulating base, said tap contacts mounted in a linear row in spaced relation on said electrically insulating base, said collector contact composed of an electrically conductive elongated strip mounted on said insulator base in spaced opposition to and flush with the row of said tap contacts and electrically isolated from said tap contacts, a framework for supporting said movable contact over said collector contact and said tap contacts, a supporting member movably supporting said framework so that it can be moved toward and away from said base, means supporting said supporting member so that it is movable over and along the row of said tap contacts for the length of said elongated collector contact when said tap changer is in the open position, and toggle means mounted on said supporting member for biasing said framework to hold said movable contact in simultaneous contact, said collector contact and the selected one of said tap contacts in a bridge like manner.
 2. Tap changer, as set forth in claim 1, wherein said housing supporting said elongated insulator base is in the form of a rectangular box and includes upstanding side walls fixed at their lower ends to said base and a top cover plate connected to the upper ends of said side walls, an upwardly extending transverse plate extending between and connecting said upstanding walls to each other, said transverse plate located between said base and said top cover plate and forming guides along its upper and lower edges, so that siad supporting plate can be moved and guided by said guides and at the same time is supported so that said supporting plate is swingable about the upper guide to some degree.
 3. Tap changer, as set forth in claim 2, wherein said top plate is formed with a plurality of slots along a longitudinal side thereof with each said slot in vertical alignment with one of said tap contacts.
 4. Tap changer as set foRth in claim 2, wherein an indexing means is associated with said supporting plate and seats into a hole in said transverse plate and is provided to facilitate the positioning of said movable contact at the one of said tap contacts to be closed.
 5. Tap changer, as set forth in claim 1, wherein an arm member secured to said supporting plate, a first shaft supported horizontally on said arm member, said toggle means comprising a bifurcated lever swingably mounted on said first shaft and a pair of link levers connected pivotably to the free end of said bifurcated levers, a second shaft supported horizontally on said framework, the opposite ends of said link levers from said bifurcated levers pivotably connected to said second shaft tension springs connecting said first and said second shafts together, and the axis of said tension springs forming a line on which the dead point of said toggle mechanism lies.
 6. Tap changer, as set forth in claim 4 wherein a toggle mechanism actuating rod is connected to said bifurcating lever and projects upwardly above said top cover plate, said actuating rod being arranged to be received in said receiving slot in the closed position of the tap changer.
 7. Tap changer as set forth in claim 1, wherein said framework comprises a pair of side plates disposed in spaced parallel relation, a web connected to said side plates and serving to guide the vertical movement of said framework along the surface of said supporting plate, means attached to said side plates and arranged to releasably suspend said bridge-like movable contact, a spring seat plate rotatably mounted on said second shaft between said side plates, and a compression spring vertically disposed between said seat plate and the upper surface of said means suspending said bridge-like movable contact bridge contact assembly.
 8. Tap changer, as set forth in claim 7, where the lower ends of said side plates are bent inwardly and the distance between said second shaft and said inwardly bent lower ends of said side plates is dimensioned so that, in the closed position of the tap changer, the suspension of said movable contact is released, whereby the said movable contact and its said suspension means assembly can be brought freely to the bridging contact position on said fixed contacts under the influence of said compression spring.
 9. Tap changer as set forth in claim 7 characterized in that the longitudinal axis of said compression spring and the vertical plane of said supporting plate are swingable to some degree around the upper edge of said supporting plate and are positioned relative to each other so that a component of force is produced by said compression spring in addition to the force to press the bridge contact assembly onto said collector and tap contacts thereby to cause said movable contact to move over the collector and tap contacts in a wiping manner when the tap changer is closed.
 10. Tap changer as set forth in claim 9, wherein a force balancing member is associated with said movable contact to equalize the pressure of the spring compression at said collector and the tap contacts. 